Electric automatic temperature control

Abstract

A temperature control circuit is disclosed utilizing automatically either the electric heating (or cooling) element or a separate sensor to measure the instantaneous temperature of the thermal load device. Either a pulsating dc or an ac electric power source may be used with the temperature sensing occurring at near-zero points of the source voltage waveform. The sensor forms one element of a Wheatstone bridge which when unbalanced by a value lower than a chosen reference value, causes a monostable oscillator to flip for at least several cycles of the source power. This turns on a power SCR to feed electric current to the thermal load to increase its temperature (and resistance) until the bridge is again balanced to reverse the state of the monostable oscillator and turn off the SCR. A simple, reliable two step, user friendly calibration circuit is provided for the temperature selecting component. For multiple channel applications the circuit need be reiterated only in part whereby major portions thereof remain common to the various channels and need not be duplicated, thereby minimizing manufacturing cost, component space, heat generation, and power consumption.

I claim: 
     
       1. Automatic temperature control and power switching circuit for a thermo-electric load comprising: AC power means for energizing the thermo-electric load, said power source means for having an alternating current waveform that periodically crosses through zero voltage;   temperature control circuit means for sensing a temperature representing that of the thermo-electric load;   power switching means responsive to said temperature control circuit means for selectively connecting said power source means to the thermo-electric load for a predetermined minimum number of half cycle waveforms selected to minimize DC bias build up due to one or several odd half cycles of conduction of one polarity of said alternating current; and   timing control circuit means connected to said power source means for preventing said switching means from operating unless said alternating current waveform of said power source means is below a predetermined amplitude.   
     
     
       2. The automatic temperature control and power switching circuit of claim 1, wherein said power switching means includes capacitive timed one shot multivibrator means for causing the connection of said power source means to said thermo-electric load for a minimum timed interval that corresponds to said predetermined minimum number of half cycle waveforms so as to minimize imbalance at said power source means, and said temperature control means having comparator means for causing connection of said power source means for a duration that depends on a load temperature sensed by said temperature control circuit means and said timing control circuit means. 
     
     
       3. The automatic temperature control and power switching circuit of claim 1, wherein said temperature control circuit means includes adjustable calibration means for independently setting a low temperature limit and a high temperature limit, and has user selection means for selecting an operating temperature within a range between said high and low temperature limits. 
     
     
       4. The automatic temperature control and power switching circuit of claim 1, further comprising at least a second switching means and a second temperature control circuit means for cooperating with a separate additional thermo-electric load; said second switching means being responsive to said second temperature control circuit means for connecting said power source means to the additional thermo-electric load, and said second switching means and said second temperature control circuit means being responsive to said timing control means so as to prevent said second switching means from operating unless the waveform of said power source is below said predetermined amplitude, whereby at least a second control channel is provided for the separate additional thermo-electric load using said power source means and said timing control means in common with the first mentioned thermo-electric load.   
     
     
       5. The automatic temperature control and power switching circuit of claim 1, wherein the thermo-electric load may be of a type using the load resistance itself to sense a temperature representing that of the load, or of the type having a separate sensor resistance for sensing the temperature representing that of the load, and further comprising: said temperature control circuit means including an auxiliary compensating resistance means; and   an adaptor-connector means including jumper means for use with one of said types of thermo-electric load for connecting in circuit said auxiliary compensating resistance, whereby said compensating resistance cooperates in said temperature control circuit means for controlling the temperature representing that of the thermo-electric load depending on the type of thermoelectric load.   
     
     
       6. The automatic temperature control and power switching circuit of claim 1, wherein said timing control means comprises: a control transistor circuit having one input connected to a regulated reference voltage and another input connected to said power source means, and having an output that disables said switching means from responding to said temperature control means when an amplitude of said waveform is above said regulated reference voltage.   
     
     
       7. The automatic temperature control and power switching circuit of claim 1, wherein the thermo-electric load is a device for melting solder. 
     
     
       8. The automatic temperature control and power switching circuit of claim 1, wherein said power switching means comprises a one-shot multivibrator means having a switch firing state triggered by said temperature control means and timing control circuit means, a timed capacitive charging state during which said power source means is connected to said thermo-electric load and a rapid discharge condition following said timed capacitive charging state for resetting said one-shot multivibrator means to enable it for refiring the next time it is triggered by said temperature control means and timing control circuit means. 
     
     
       9. Automatic temperature control and power switching circuit for a thermo-electric load, comprising; power source means having an alternating current or pulsed DC waveform that periodically approaches zero voltage;   temperature control circuit means for sensing a temperature representing that of the thermo-electric load, said temperature control circuit means including means for independently setting high and low temperature range limits;   switching means responsive to said temperature control circuit means for selectively connecting said power source means to the thermo-electric load wherein the duration of such connection is controlled by said temperature control circuit means; and   timing control means connected to said switching means for preventing said switching means from operating unless the waveform of said power source means is below a predetermined level.   
     
     
       10. The automatic temperature control and power switching circuit of claim 9, wherein said means for independently setting said high and low temperature range limits in said temperature control circuit means comprises an operational amplifier having an adjustable reference level at a low impedance output, and a user selected temperature setting potentiometer in a resistive bridge network, said low impedance operational amplifier output being connected to said bridge network adjacent said potentiometer to establish one of said temperature range limits. 
     
     
       11. The automatic temperature control and power switching circuit of claim 9, wherein said means in said temperature control circuit means for independently setting high and low temperature range limits comprises: resistive bridge network means for comparing a resistance that varies with the temperature representing that of said load with a user adjustable potentiometer resistance to select a temperature setting;   low temperature calibration means including an operational amplifier having an adjustable input level and a low impedance output that is coupled to said resistive bridge network so as to establish a low temperature calibration reference level for said potentiometer; and   high temperature limit calibration means including a variable resistance means connected in said bridge network for setting the high temperature range limit, whereby said operational amplifier output of said low temperature calibration means establishes a reference level at one end of said temperature setting potentiometer that remains independent of changes of the high temperature calibration means.   
     
     
       12. The automatic temperature control and power switching circuit of claim 11, further comprising an operational amplifier comparator having inputs connected to said resistive bridge network means and having an output that changes in a discrete voltage step when the temperature representing that of the load falls below the user selected temperature. 
     
     
       13. The automatic temperature control and power switching circuit of claim 11, wherein the thermo-electric load is a soldering device. 
     
     
       14. Automatic temperature control and power switching circuit for a plurality of thermo-electric loads comprising: power source means for energizing the thermo-electric load, said power source means having an alternating current or pulsed DC waveform that periodically approaches zero voltage;   first and second temperature control circuit means for sensing temperatures respectively representing that of first and second thermo-electric loads;   first and second power switching means respectively responsive to said first and second temperature control circuit means for selectively connecting said power source means to the first and second thermo-electric loads; and   timing control circuit means connected to said power source means for preventing said switching means from operating unless said waveform of said power source means is below a predetermined amplitude.   
     
     
       15. The automatic temperature control and power switching circuit of claim 14, wherein said plurality of thermo-electric loads are separate soldering devices. 
     
     
       16. Automatic temperature control and power switching circuit for a plurality of thermo-electric loads comprising: common alternating current power source means for energizing the thermo-electric loads, said power source means having an alternating current waveform that periodically crosses through zero voltage;   first temperature control circuit means for sensing a temperature representing that of a first thermo-electric load, said temperature control circuit means including means for independently setting high and low calibration temperature limits;   first power switching means responsive to said first temperature control circuit means for selectively connecting said common alternating current power source means to a first thermo-electric load for a predetermined minimum number of half cycle waveform;   second temperature control circuit means for sensing a temperature representing that of a second thermo-electric load, said second temperature control circuit means including means for independently setting high and low calibration temperature limits;   second power switching means responsive to said second temperature control circuit means for selectively connecting said common alternating current power source means to a first thermo-electric load for a predetermined minimum number of half cycle waveform; and   common timing control circuit means connected to said common alternating current power source means for preventing said first and second switching means from operating unless said waveform of said power source means is below a predetermined amplitude.   
     
     
       17. The automatic temperature control and power switching circuit of claim 15, wherein the thermo-electric loads are soldering devices. 
     
     
       18. The automatic temperature control and power switching circuit of claim 17, wherein the thermo-electric loads of the soldering devices may be either of a type using the load resistance itself to sense a temperature representing that of the device, or of the type having a separate sensor resistance for sensing the temperature representing that of the device, and further comprising: at least one of said first and second said temperature control circuit means including an auxiliary compensating resistance means; and   an adaptor-connector means including jumper means for use with one of said types of thermo-electric load soldering devices for connecting in circuit said auxiliary compensating resistance, whereby said compensating resistance cooperates in said one of said first and second temperature control circuit means for controlling the temperature representing that of the thermo-electric load depending on the type of thermo-electric load soldering device.   
     
     
       19. The automatic temperature control and power switching circuit of claim 16, wherein said common alternating current power source means comprises an unfiltered rectified AC supply and further comprising a filtering and regulating network means coupled to said unfiltered rectified AC supply for providing a DC control voltage supply. 
     
     
       20. The automatic temperature control and power switching circuit of claim 16, wherein said first and second power switching means each comprise a solid state switching device having a control lead coupled by circuit means to an output of said respective first and second temperature control circuit means.